This invention relates to soldering components on printed circuit boards, and more specifically to preheating prior to soldering.
Some PCB's do not have through holes and these are known as surface mount devices (SMD's). A preferred soldering technique for SMD's is infrared (IR) reflow in which solder paste on the PCB's is caused to reflow upon the application of heat. More particularly, heating takes place in separately controlled zones within the machine permitting precise temperature profiling. The boards pass by conveyor first into a preheat zone which gently heats the solder paste to drive off volatile solvents and avoid thermal shock to the PCB's. After preheating, the PCB's pass into the main heating zones where they are raised to reflow temperature. On exiting from the heating zones the boards are cooled, typically by blowing air up through the conveyor mesh, to solidify the solder joints.
In a development of this technique, automatic soldering machines have been designed in which the IR heating is supplemented with forced convection. In one such machine marketed by the assignee of the present application, blowers are provided above a top row of heaters which are perforated so that hot air is blown on to the PCB's.
So as to reduce or eliminate problems caused by oxidation of the solder paste it has been proposed that, instead of air, an inert gas such as nitrogen should be used for the atmosphere inside the heating zones and that forced convection would involve blowing the inert gas rather than air. Inert gas as used herein includes a gas that excludes oxygen, and nitrogen is considered an inert gas for this application.
In another embodiment PCB's are preheated prior to being wave soldered with or without a fluxing step. Fluxing occurs prior to the preheating step and generally permits soldering in air although an inert gas atmosphere may be used in either or both the preheating and the wave soldering steps. When fluxing does not occur, the preheating may be in air or inert gas, but the wave soldering is generally in inert gas.
As electronic components become more complex and massive, they are difficult to preheat uniformly and require special preheating prior to wave soldering. This special preheating is required for both fluxing and fluxless soldering. In the past preheating has occurred with infrared heaters which generally are used to heat only the bottom of circuit assemblies prior to wave soldering. Although convection preheaters have been provided on the top and bottom of conveyors in preheat zones to provide proper heat profiling for reflow soldering, they have only occasionally been tried for wave soldering, and are not presently used.